This invention relates to an inkjet ink composition for use in an inkjet recording apparatus for conducting recording on plain paper at a high speed.
As an ink composition for use in inkjet recording, there have so far been widely used water-soluble liquid ink compositions. However, when printing is conducted on plain paper popularly used in offices such as copying paper, the printed ink spreads along fibers of the paper, thus shape of dots becoming irregular to generate so-called blurring which deteriorates quality of printed products.
Various improvements have so far been made in order to remove blurring and improve drying properties. For example, Japanese Patent Publication No. 34992/1985 proposes to reduce surface tension of ink and enhance drying properties of ink by using a large quantity of a surface active agent. Japanese Patent Laid-Open No. 108271/1983 describes a method of recording by using a hot-melt ink composition containing a substance which is solid at room temperature, such as wax. Further, U.S. Pat. Nos. 4,391,369 and 4,484,948 describe an ink composition which provide good printing quality regardless of the properties of paper.
It has also been attempted to improve ink compositions themselves. As a water-soluble liquid ink composition, Japanese Patent Laid-Open No. 233781/1990 describes a recording liquid of a water-soluble medium and a dye containing as a humectant a water-soluble organic solvent composed of diethylene glycol, glycerin and polyethylene glycol in a content of 4 to 20 wt %. Japanese Patent Publication No. 33110/1995 discloses an ink containing as necessary components a dye, a volatile solvent and a solid substance having a molecular weight of 300 or more. Japanese Patent No. 2,516,218 discloses a recording method of printing on paper of 10 or more in Stxc3x6ckigt sizing degree using a composition containing a dye, a wetting agent and water and having a dynamic surface tension and a viscosity the sum of which is 42 to 49. Japanese Patent No. 2,801,295 discloses a method of recording on a recording medium of 0 to 15 in Stxc3x6ckigt sizing degree by optimizing the dye concentration Further, Japanese Patent Laid-Open No. 330666/1998 discloses to use a recording medium of 10 or more in Stxc3x6ckigt sizing degree for evaporation of aqueous inks. Japanese Patent No. 2,529,154 shows an inkjet composition using a non-aqueous solvent showing an evaporation rate of {fraction (1/10)} that of butyl acetate.
On the other hand, in order to improve weatherability of printed products, pigments are generally used as colorants of inks for use in various printers such as laser printers, melt transfer printers, liquid inkjet printers and solid inkjet printers.
As to pigment inks, many reports have been made by, for example, Japanese Patent Laid-Open Nos. 37278/1991, 339871/1992, 16343/1993, 105832/1993, 49400/1994, 228479/1994, 228480/1994, 306319/1994, 109432/1995, 196968/1995, 278477/1995, 306319/1995, 316479/1995, 331141/1995 and 295836/1996.
However, most of popularly used recording liquids for inkjet recording have been recording liquids using an aqueous medium and a dye. For, in case where pigments are used, it is difficult to combine constituents so as to stably keep the pigments in a dispersed state, and thus keeping the recording liquids for a long time is difficult.
As is different from those recording liquids in which dyes soluble in an aqueous medium are used, recording liquids using pigments are liable to cause clogging of a nozzle pore of a recording head since the pigments are insoluble in an aqueous medium and therefore in a solid state, thus a large quantity of humectant such as polyalkylene glycol or glycerin becoming necessary.
For example, Japanese Patent Laid-Open No. 157464/1991 discloses a recording liquid using a pigment, in which a humectant is used in an amount of preferably 30 to 80 parts per 100 parts by weight of water. It has also been tried to add a large amount of a humectant in order to prevent aggregation of pigments, but such addition causes an increase in viscosity of the recording liquid so much that ejecting properties of the liquid is decreased. In order to improve drying resistance at the tip of a pen without increasing viscosity, Japanese Patent Publication No. 21250/1994 discloses an aqueous pigment ink containing trimethylpentanediol monoisobutylate and ethylene glycol monobutyl ether. Although this Japanese Patent Publication No. 21250/1994 discloses a recording liquid showing a high fixing rate by incorporating dialkyl ether, every Example therein shows an ink viscosity of as low as about 3 mPaxc2x7s. Japanese Patent Laid-Open No. 209046/1996 describes that a recording liquid containing 8 to 15% by weight of a water-soluble organic solvent is preferred, said solvent containing as a humectant polyethylene glycol having a molecular weight of 100 to 500. However, it does not describe specific viscosity values and materials to be recorded. In addition, Japanese Patent Laid-Open No. 199807/1999 discloses a recording liquid which contains 1.5 to 3% by weight of a water-insoluble or slightly water-soluble colorant and 2 to 7.5% by weight of polyalkylene glycol having a weight average molecular weight of 2000 to 20000. Japanese Patent Laid-Open No. 228884/1999 describes an ink comprising a pigment, a humectant and an aqueous medium, which is characterized in that the amount of residual water is 0.5 to 10 g per 100 g of the ink. However, it does not describe the amount of initial water, and does not refer to printing quality with respect to ejecting properties.
Japanese Patent Publication No. 37834/1985 discloses a recording liquid using in combination polyalkylene glycol of 150 to 350 in an average molecular weight and polyalkylene glycol of 400 to 750 in an average molecular weight mainly for obtaining a recording liquid having excellent ejection stability, ejection response and continuously recording ability. However, it does not describe other advantages, nor to use polyalkylene glycol having a high molecular weight.
Further, in the above-described publication, no consideration is given about improvement of jet stability and quality of printed produces by the viscosity-increasing effect of polyethylene glycol and glycol ether in aqueous inks. A marked water viscosity-increasing effect is obtained by polyethylene glycol and/or glycol ether, and the use of only polyhydric alcohol humectants such as glycerin fails to attain the main object of the present invention. That is, function of polyethylene glycol in the ink composition of the present invention is greatly different from that conventionally used as humectant.
When used for printing, aqueous inks are difficultly printed on recording materials with no ink-absorbing properties and, even when paper for exclusive use is used, a large-sized ink drying apparatus is required. In addition, since highly fine printing is difficult due to blurring problem which sets a limit to resolving power, application of such inks is limited. In some cases, solvent inks are used in view of their quick-drying advantage. However, they are not desirable due to the problems of combustion and environmental pollution. Hot-melt oily inks enable high speed printing but, due to the essential properties of the inks, printed dots have a thickness of about 10 to 20 xcexcm at the thickest, and hence they provide such a poor durability that it has been difficult to obtain enough reliability of printed products.
Inkjet recording system using organic pigments as colorants have much more advantages than inkjet recording system using dyes especially with respect to weatherability. Hence, the inkjet recording system is expected to be applied not only printers such as OA instruments, home printers and facsimiles but other uses such as outdoor or indoor posters, large-sized signboards, decoration of cars, glasses, elevators, walls and buildings, and printing of cloths. However, as has been described hereinbefore, the problem of lack of highly relaiable printing quality has been a bottleneck to commercialization of the system.
An object of the present invention is to provide an ink composition for use in an inkjet printer capable of giving recorded products of good quality on plain paper through high-speed drying, and a method of printing the ink composition.
Another object of the present invention is to provide a pigment ink composition for a high speed inkjet printer, which shows enough excellent ejecting stability to jet through fine nozzles in the form of ink droplets.
A still other object of the present invention is to provide a recording process suitable for high speed recording.
Other objects and effects of the invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing the following ink compositions and printing processes.
(1) An ink composition for use in an on-demand inkjet printer capable of printing on plain paper at a recording rate of 10 ips or higher, which ink composition comprises at least a pigment and non-pigment components including at least water and a thickening agent and shows a weight loss ratio at 25xc2x0 C. and 60 RH (relative humidity) % for 30 minutes of 5 to 25% based on the total weight of the ink
(2) The ink composition as described in item (1) above, wherein said non-pigment components comprises:
at least one of 7.5 to 40% by weight of a polyethylene glycol having an average molecular weight of 600 to 6,000 and 1 to 10% by weight of a polyethylene glycol having an average molecular weight of 20,000 to 70,000 as a thickening agent;
0 to 30% by weight of at least one of an ethylene glycol ether and a propylene glycol ether; and
0 to 30% by weight of glycerin.
(3) The ink composition as described in item (1) or (2) above, having a viscosity at 25xc2x0 C. of 5 to 15 mPaxc2x7s and a surface tension of 30 to 55 mN/m.
(4) An on-demand inkjet recording process comprising:
recording an image on plain paper by ejecting an ink composition at a recording rate of 10 ips or higher,
wherein said ink composition is as described in item (1) above.
(5) The process as described in item (4) above, wherein wherein said non-pigment components comprises:
at least one of 7.5 to 40% by weight of a polyethylene glycol having an average molecular weight of 600 to 6,000 and 1 to 10% by weight of a polyethylene glycol having an average molecular weight of 20,000 to 70,000 as a thickening agent;
0 to 30% by weight of at least one of an ethylene glycol ether and a propylene glycol ether; and
0 to 30% by weight of glycerin.
(6) The process as described in item (4) or (5) above, wherein said ink composition has a viscosity at 25xc2x0 C. of 5 to 15 mPaxc2x7s and a surface tension of 30 to 55 mN/m.
(7) The process as described in any one of items (4) to (6) above, wherein said plain paper has a Stxc3x6ckigt sizing degree of 55 or less.